– How do scientists know that a sea creature is extinct or endangered if they haven’t explored 80% of the sea?
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That “we haven’t explored 80%” is very misleading. We haven’t visited 80%, but we have scanned loads of it.
And we can never be sure if an animal is really extinct. That’s an estimation we make based on how often we see something. I.E. the last 30 years we saw 100 spiral headed snails per survey on average, this year we saw only 1 per survey, so it’s very likely that the species isn’t doing well.
Proving things don’t exist tends to be very hard, or flat out impossible.
In the case of an extinct animal, you’d have to check every single possible location it could possibly be at. You’d also have to check them all at the same time to ensure the animal didn’t just move to a location you already checked. Basically, it’s impossible.
So science just takes the reasonable approach and goes “this animal hasn’t been seen for X years, so it’s probably extinct. That X goes up or down depending on how likely it should be for someone to have seen the animal.
Is it a big, diurnal animal that lives in densely populated areas? Probably should be spotted fairly often, so going a couple years without a single sighting means something. On the other hand, is it some deep sea creature that lives in an habitat humans never enter? It’s probably normal to go decades without seeing one.
We don’t. As an example we thought the Coelacanth went extinct millions of years ago until we found them off the coast of Africa.
https://en.m.wikipedia.org/wiki/Coelacanth
In addition to the other comments regarding surveys and scientific probabilities, you also bear in mind that most species have a specific habitat, or certain habitat parameters and requirements.
For example, let’s say there is a reef fish that needs a specific temperature range; a specific group of 3-5 plants it eats; and it needs a certain type of coral it lays its eggs in.
Now let’s say those types of reefs are deeply damaged somehow – the temperature of the water goes up. The plants die back. The coral becomes ‘bleached’. As the reef dies, so does our fish species.
So in general, my point is that marine life can’t just live anywhere. Most species have specific habitats and 95% of the ocean doesn’t offer the 5% critical to the species in question.
The world can be divided into “biomes” which share characteristics such as rainfall, temperature, sunlight etc. and most life is suited to one particular biome.
Conditions in the oceans vary massively with depth so a creature living near the surface won’t be found in the unexplored depths of the seas, thus we can conclude that a creature we know about with a declining population is endangered and won’t be found in deeper water.
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